Defects in liquid crystals

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Abstract
The most striking feature of liquid crystals is the wide variety of visual patterns they display. These patterns, such as those shown in figure 1 and on the cover, are due almost entirely to the defect structure that occurs in the long-range molecular order of the liquid. Indeed, historically, the underlying structure of the liquid-crystal phases known as nematic and smectic-A was discovered from a study of stable defects that characterize these phases. Such defects are easily visible in the optical microscope. By examining the thin and thick thread-like structures observed in nematic liquid crystals, Otto Lehman and Georges Friedel deduced that this phase involves long-range orientational ordering of the long axis of the rod-like molecules. (The direction of this orientational ordering is what we now denote by the unit vector n̂, called the director.)